Backside refraction layer for backside illuminated image sensor and methods of forming the same

ABSTRACT

Photosensors may be formed on a front side of a semiconductor substrate. An optical refraction layer having a first refractive index may be formed on a backside of the semiconductor substrate. A grid structure including openings is formed over the optical refraction layer. A masking material layer is formed over the grid structure and the optical refraction layer. The masking material layer may be anisotropically etched using an anisotropic etch process that collaterally etches a material of the optical refraction layer and forms non-planar distal surface portions including random protrusions on physically exposed portions of the optical refraction layer. An optically transparent layer having a second refractive index that is different from the first refractive index may be formed on the non-planar distal surface portions of the optical refraction layer. A refractive interface refracts incident light in random directions, and improves quantum efficiency of the photosensors.

BACKGROUND

The present disclosure is directed to semiconductor devices, andspecifically to a backside illuminated complementarymetal-oxide-semiconductor (CMOS) image sensor and methods of forming thesame.

Semiconductor image sensors are used to sense electromagnetic radiationsuch as visible range light, infrared radiation, and/or ultravioletlight. Complementary metal-oxide-semiconductor (CMOS) image sensors(CIS) and charge-coupled device (CCD) sensors are used in variousapplications such as digital cameras or embedded cameras in mobiledevices. These devices utilize an array of pixels (which may includephotodiodes and transistors) to detect radiation using photogenerationof electron-hole pairs. A backside illuminated (BSI) image sensor is animage sensor that is configured to detects light impinging on a backsideof a semiconductor substrate. CMOS circuitry for detecting andprocessing photogenerated signals may be formed on a front side of thesemiconductor substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIG. 1A is a plan view of a first configuration for an array of pixelsof an image sensor according to an embodiment of the present disclosure.

FIG. 1B is a plan view of a second configuration for an array of pixelsof an image sensor according to an embodiment of the present disclosure.

FIG. 2A is a plan view of front side sensor components within the areaof a subpixel in an exemplary structure according to an embodiment ofthe present disclosure.

FIG. 2B is a vertical cross-sectional view of the exemplary structurealong the vertical plane B-B′ of FIG. 2A.

FIG. 3 is a vertical cross-sectional view of the exemplary structureafter formation of metal interconnect structures formed ininterconnection-level dielectric layers and attachment of a carriersubstrate according to an embodiment of the present disclosure.

FIG. 4 is a vertical cross-sectional view of the exemplary structureafter thinning a semiconductor substrate according to an embodiment ofthe present disclosure.

FIG. 5 is a vertical cross-sectional view of the exemplary structureafter formation of deep trenches on the backside of the semiconductorsubstrate according to an embodiment of the present disclosure.

FIG. 6 is a vertical cross-sectional view of the exemplary structureafter removal of a hard mask layer and a pad dielectric layer accordingto an embodiment of the present disclosure.

FIG. 7 is a vertical cross-sectional view of the exemplary structureafter formation of a dielectric metal oxide liner and a dielectricisolation layer according to an embodiment of the present disclosure.

FIG. 8 is a vertical cross-sectional view of the exemplary structureafter formation of deep trench isolation structures according to anembodiment of the present disclosure.

FIG. 9 is a vertical cross-sectional view of the exemplary structureafter formation of an antireflective coating (ARC) layer, an opticalrefraction layer, a dielectric grid material layer, a metallicreflective material layer, and a patterned photoresist layer accordingto an embodiment of the present disclosure.

FIG. 10A is a vertical cross-sectional view of the exemplary structureafter formation of a composite grid structure according to an embodimentof the present disclosure.

FIG. 10B is a plan view of the exemplary structure of FIG. 10A. Thehinged vertical plane A-A′ corresponds to the plane of the verticalcross-sectional view of FIG. 10A.

FIG. 11A is a vertical cross-sectional view of the exemplary structureafter formation of a masking material layer and a patterned photoresistlayer according to an embodiment of the present disclosure.

FIG. 11B is a plan view of the exemplary structure of FIG. 11A. Thehinged vertical plane A-A′ corresponds to the plane of the verticalcross-sectional view of FIG. 11A.

FIG. 11C is a vertical cross-sectional view of the exemplary structurealong the vertical plane C-C′ of FIG. 11B.

FIG. 12A is a vertical cross-sectional view of the exemplary structureafter formation of non-planar distal surface portions on the opticalrefraction layer by anisotropically etching the masking material layerand collaterally etching top portions of the optical refraction layeraccording to an embodiment of the present disclosure.

FIG. 12B is a plan view of the exemplary structure of FIG. 12A. Thehinged vertical plane A-A′ corresponds to the plane of the verticalcross-sectional view of FIG. 12A.

FIG. 12C is a vertical cross-sectional view of the exemplary structurealong the vertical plane C-C′ of FIG. 12B.

FIG. 12D is a scanning electron micrograph (SEM) of a top surface of anoptical refraction layer of a test sample according to an embodiment ofthe present disclosure.

FIG. 13A is a vertical cross-sectional view of the exemplary structureafter removal of the patterned photoresist layer according to anembodiment of the present disclosure.

FIG. 13B is a plan view of the exemplary structure of FIG. 13A. Thehinged vertical plane A-A′ corresponds to the plane of the verticalcross-sectional view of FIG. 13A.

FIG. 13C is a vertical cross-sectional view of the exemplary structurealong the vertical plane C-C′ of FIG. 13B.

FIG. 14 is a vertical cross-sectional view of the exemplary structureafter formation of an optically transparent layer, color filters, andlenses according to an embodiment of the present disclosure.

FIG. 15A is a vertical cross-sectional view of the exemplary structureafter removal of the carrier substrate according to an embodiment of thepresent disclosure.

FIG. 15B is another vertical cross-sectional view of the exemplarystructure of FIG. 15A.

FIG. 16 is a flow chart for an exemplary process sequence for forming animage sensor according to an embodiment of the present disclosure.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the provided subjectmatter. Specific examples of components and arrangements are describedbelow to simplify the present disclosure. These are, of course, merelyexamples and are not intended to be limiting. For example, the formationof a first feature over or on a second feature in the description thatfollows may include embodiments in which the first and second featuresare formed in direct contact, and may also include embodiments in whichadditional features may be formed between the first and second features,such that the first and second features may not be in direct contact. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly.

Generally, the structures and methods of the present disclosure may beused to provide an image sensor having a higher quantum efficiency,which is the fraction of photons that produces an electron-hole pairamong all photons that impinge into a photo-reactive material portion.An optical refraction layer having random surface roughness is used toprovide random refraction of photons that impinge on a photosensor,which increases the quantum efficiency of the photosensor.

A refractive structure in the optical path of photons within an imagesensor may have a beneficial effect by increasing the probability ofphoton absorption in a photosensor. For example, Z. Y. Wang et al.,Broadband optical absorption by tunable Mie resonances in siliconnanocone arrays, Scientific Reports, Vol. 5, Article number: 7810 (2015)describes how an array of refractive pillar structures may increase theabsorption coefficient for photons of various wavelengths in the opticalrange. However, manufacture of such a periodic array of pillars requireslithographic patterning of periodic structures using deep ultraviolet(DUV) lithography, and thus may be a costly process. Embodiments of thepresent disclosure provide a randomly refractive surface having lateraldimensions on the order of 100 nm by using an anisotropic etch processthat produces a randomly refractive surface including randomprotrusions. Process non-uniformity that provides local variations inthe etch thickness in a masking material layer is amplified in anunderlying optically transparent material by using an etch chemistrythat etches the optically transparent material at a greater etch ratethan the material of the masking material layer. The resulting randomlyrefractive surface is provided in the optical path of impinging photonswithin an image sensor to enhance the quantum efficiency of the imagesensor.

Referring to FIGS. 1A and 1B, a first configuration for an array 1000 ofpixels 900 of an image sensor and a second configuration of an array1000 of pixels 900 of an image sensor are illustrated in a plan view,respectively. The image sensor may be a backside illuminated (BSI) imagesensor device. However, it should be appreciated that embodiments of thedisclosure may be used in a front side illuminated (FSI) image sensor.

Each pixel 900 represents a smallest unit area for the purpose ofgenerating an image from the image sensor. The region including thearray 1000 of pixels 900 is herein referred to as a pixel array region.The pixels 900 in the pixel array region may be arranged in rows andcolumns. For example, the pixel array region may include M rows and Ncolumns, in which M and N are integers in a range from 1 to 2¹⁶, such asfrom 2⁸ to 2¹⁴. The rows of pixels 900 may be consecutively numberedwith integers that range from 1 to M, and the columns of pixels 900 maybe consecutively numbered with integers that range from 1 to N. A pixelPij refers to a pixel 900 in the i-th row and in the j-th column.

Each pixel 900 includes at least one photosensor that is configured todetect radiation of a given wavelength range. Each pixel 900 may includea plurality of photosensors configured to detect radiation of arespective wavelength range, which may be different among the pluralityof photosensors. In one embodiment, each pixel 900 may include aplurality of subpixels, each of which including a respective combinationof a photosensor and an electronic circuit configured to detectradiation that impinged into the photosensor. For example, a pixel 900may include a subpixel configured to detect radiation in a redwavelength range (such as a range from 635 nm to 700 nm), a subpixelconfigured to detect radiation in a green wavelength range (such as arange from 520 nm to 560 nm), and a subpixel configured to detectradiation in a blue wavelength range (such as a range from 450 nm to 490nm). Such subpixels are referred to as a red subpixel, green subpixel,and a blue subpixel, respectively.

Generally, a pixel 900 generates information in impinging radiation fora unit detection area. A subpixel generates information on the intensityof the impinging radiation within a specific wavelength range asdetected within a region of the unit detection area. A monochromaticpixel 900 may include only a single subpixel. A pixel 900 configured todetect spectral distribution of impinging radiation includes multiplesubpixels having at least two different detection wavelength ranges.Photosensors in a pixel array region may include photodiodes,complimentary metal-oxide-semiconductor (CMOS) image sensors, chargedcoupling device (CCD) sensors, active sensors, passive sensors, otherapplicable sensors, or a combination thereof.

Referring to FIGS. 2A and 2B, front side sensor components 600 withinthe areas of a subpixel in an image sensor are illustrated. Asemiconductor substrate 500 includes a substrate semiconductor layer601. Front side sensor components 600 include all components of theimage sensor that may be formed on the front surface 609 of thesemiconductor substrate 500, or may be formed within the substratesemiconductor layer 601. Each subpixel includes a photosensor and asensing circuit for the photosensor. A set of subpixels may be used fora pixel, and an array 1000 of pixels may be arranged as illustrated inFIG. 1A or in FIG. 1B, or in any other suitable array configurations toprovide an image sensor.

Each subpixel may be formed on, or in, the substrate semiconductor layer601, which has a front surface 609 and a back surface. The substratesemiconductor layer 601 includes a semiconductor material such assilicon, germanium, a silicon-germanium alloy, a compound semiconductormaterial, or any other semiconductor material having a band gap thatthat does not exceed the energy of the photons to be detected. Thematerial within the substrate semiconductor layer 601 may be selectedbased on the energy range of the photons to be detected by the subpixel.In one embodiment, the substrate semiconductor layer 601 may includesingle crystalline silicon. A commercially available single crystallinesemiconductor substrate may be used for the semiconductor substrate 500.The semiconductor substrate 500 as provided at this processing step hasa sufficiently high thickness to be able to withstand standardcomplementary metal-oxide-semiconductor (CMOS) processing steps. Forexample, the thickness of the semiconductor substrate 500 may be in arange from 200 microns to 1 mm, although lesser and greater thicknessesmay also be used.

A top portion of the substrate semiconductor layer 601 may be suitabledoped to have a first conductivity type, which may be p-type or n-type.For example, an epitaxial semiconductor deposition process may beperformed to form a single crystalline epitaxial semiconductor materiallayer at an upper portion of the substrate semiconductor layer such thatthe atomic concentration of the dopants of the first conductivity typeis in a range from 1.0×10¹³/cm³ to 1.0×10¹⁶/cm³, although lesser andgreater atomic concentrations may also be used. The thickness of thesingle crystalline epitaxial semiconductor material layer may be in arange from 1 micron to 10 microns.

First-conductivity-type wells 607 may be formed by ion implantationaround regions in which shallow trench isolation structures 620 may besubsequently formed. The atomic concentration of dopants of the firstconductivity type in the first-conductivity-type wells 607 may be in arange from 1.0×10¹⁵/cm³ to 1.0×10¹⁸/cm³, although lesser and greateratomic concentrations may also be used. Shallow trench isolationstructures 620 may be formed to provide electrical isolation from thevarious components within the subpixel.

Gate structures (614, 605, 615) may be formed over the front surface 609of the semiconductor substrate 500 by depositing and patterning a layerstack including a gate dielectric layer and a gate electrode layer. Eachpatterned portion of the layer stack constitutes a gate structure (614,605, 615), which include a transfer gate structure (614, 605) andcontrol gate structures (614, 615). The transfer gate structure (614,605) is the gate structure for a transfer transistor 630, and includes astack of a gate dielectric 614 and a transfer gate electrode 605. Eachof the control gate structures (614, 615) includes a respective layerstack of a gate dielectric 614 and a gate electrode 615 of othertransistors in a sensing circuit, which may include a reset transistor640, a source-follower transistor 650, a select transistor 660, andother suitable transistors that may be used to amplify the signalgenerated by the photosensor of the subpixel.

Dopants of a second conductivity type may be implanted through the frontsurface 609 of the semiconductor substrate 500 using at least one maskedion implantation process. The second conductivity type is the oppositeof the first conductivity type. For example, if the first conductivitytype is p-type, the second conductivity type is n-type, and vice versa.Various doped regions having a doping of the second conductivity typemay be formed by the at least one masked ion implantation process. Asecond-conductivity-type pinned photodiode layer 602 may be formedunderneath the front surface 609 of the semiconductor substrate 500 suchthat a periphery of the second-conductivity-type pinned photodiode layer602 overlaps with an edge of the transfer gate electrode 605 in a planview. Various active regions (608, 612) having a doping of the secondconductivity type may be formed, which include a floating diffusionregion 608 that functions as the drain region of the transfer transistor630. The second-conductivity-type pinned photodiode layer 602accumulates electrical charges (such as electrons in case the secondconductivity type is n-type) during sensing (i.e., while the subpixelactively detects the photons impinging thereupon, for example, for thepurpose of taking a frame or a photo) functions as the source region ofthe transfer transistor 630. The active regions 612 include sourceregions and drain regions of the various transistors (640, 650, 660) inthe sensing circuit.

A first-conductivity-type pinning layer 603 may be formed directly ontop of the second-conductivity-type pinned photodiode layer 602 by ionimplantation of dopants of the first conductivity type. Thefirst-conductivity-type pinning layer 603 may suppress depletion of theinterface between the second-conductivity-type pinned photodiode layer602 and the first-conductivity-type pinning layer 603, and electricallystabilizes the second-conductivity-type pinned photodiode layer 602.

Interconnect-level dielectric layers 670 may be formed over the frontsurface 609 of the semiconductor substrate 500, and metal interconnectstructures 680 (including metal line structure 684 and metal viastructure 682) connecting the various nodes of the transistors (630,640, 650, 660) may be formed within each subpixel. Theinterconnect-level dielectric layers 670 may include a respectivedielectric material such as undoped silicate glass, a doped silicateglass, organosilicate glass, a porous dielectric material, orcombinations thereof. Dielectric liners including various dielectricmaterials (such as silicon nitride, silicon oxynitride, silicon oxidecarbide, and/or dielectric metal oxides) may be optionally used in theinterconnect-level dielectric layers 670. The metal interconnectstructures 680 may include various metal via structures 682 and variousmetal line structures 684. For example, the floating diffusion region608 may be connected to the gate electrode 615 of the source-followertransistor 650 by a subset of the metal interconnect structures 680. Aphotosensor may comprise a transfer transistor 630, and may be connectedto a sense circuit including additional transistors (640, 650, 660).

Referring to FIG. 3 , additional interconnect-level dielectric layers670 and additional metal interconnect structures 680 may be formed onthe front side of the semiconductor substrate 500. The front side of theassembly of the semiconductor substrate 500, the interconnect-leveldielectric layers 670, and the structures formed therein may be bondedto a carrier substrate 690. The carrier substrate 690 may be temporarilyattached to the assembly of the semiconductor substrate 500 and theinterconnect-level dielectric layers 670 to enable subsequent thinningof the semiconductor substrate 500, and to enable subsequent handling ofthe assembly of a thinned semiconductor substrate 500 and theinterconnect-level dielectric layers 670. The carrier substrate 690 mayinclude a semiconductor material, an insulating material, or a metallicmaterial, and may have a thickness in a range from 300 microns to 1 mm,although lesser and greater thicknesses may also be used.

Any suitable bonding method may be used to bond the carrier substrate690 to the front side of the interconnect-level dielectric layers 670.Exemplary bonding methods that may be used to bond the carrier substrate690 to the interconnect-level dielectric layers 670 include, but are notlimited to, oxide-to-oxide bonding, oxide-to-semiconductor bonding,fusion bonding, hybrid bonding, anodic bonding, direct bonding, othersuitable bonding processes, and/or combinations thereof. Other suitablebonding techniques within the contemplated scope of disclosure may beused. Optionally, a bonding buffer layer 689 including an intermediatebonding material (e.g., silicon oxide, silicon nitride, or asemiconductor material) may be used to provide bonding between theinterconnection-level dielectric layers 670 and the carrier substrate690.

Referring to FIG. 4 , the backside of the semiconductor substrate 500may be thinned, for example, by grinding, polishing, an isotropic etchprocess, and/or an anisotropic etch process. The carrier substrate 690may provide mechanical support to the semiconductor substrate 500 duringthe thinning process. In one embodiment, the semiconductor substrate 500may be thinned to a thickness in a range from 1 micron to 12 microns,such as from 1.5 microns to 8 microns. The semiconductor substrate 500as thinned after the thinning process is herein referred to as a thinnedsemiconductor substrate 510, or as a semiconductor substrate 500. Thethickness of the thinned semiconductor substrate 510 may be determinedby the maximum depth of deep trenches to be subsequently formed on thebackside of the thinned semiconductor substrate 510. In one embodiment,the thickness of the thinned semiconductor substrate 510 may be selectedsuch that deep trenches to be subsequently formed on the backside of thesemiconductor substrate 510 reaches proximal surfaces of the shallowtrench isolation structures 620. The backside surface 709 of the thinnedsemiconductor substrate 510 may be polished to provide a planarhorizontal surface that is parallel to the front surface 609 of thethinned semiconductor substrate 510. The exemplary structure may besubsequently flipped upside down for further processing.

Referring to FIG. 5 , an optional pad dielectric layer 711 and a hardmask layer 712 may be formed over the backside surface 709 of thesemiconductor substrate 510. The optional pad dielectric layer 711, ifpresent, may include a silicon oxide layer, and may have a thickness ina range from 5 nm to 50 nm. The hard mask layer 712 includes an etchmask material that may be subsequently removed selective to the paddielectric layer 711 and/or selective to the semiconductor substrate510. For example, the hard mask layer 712 may include silicon nitride,borosilicate glass, or a metallic material. The hard mask layer 712 mayhave a thickness in a range from 50 nm to 800 nm, although lesser andgreater thicknesses may also be used.

A photoresist layer (not shown) may be applied over the hard mask layer712, and may be lithographically patterned to form openings thatgenerally replicate the pattern of the shallow trench isolationstructures 620 located underneath. A first etch process may be performedto transfer the pattern in the photoresist layer through the hard masklayer 712 and the optional pad dielectric layer 711. Unmasked portionsof the semiconductor substrate 510 may be etched by performing a secondanisotropic etch process, which transfers the pattern of the openings inthe photoresist layer and the hard mask layer 712 through thesemiconductor substrate 510 to a top surface of a respective one of theshallow trench isolation structures 620. The depth of the deep trenches719 may be in a range from 1 micron to 10 microns, such as from 1.5microns to 8 microns. The photoresist layer may be completely consumedduring the second anisotropic etch process. Deep trenches 719 may beformed through the semiconductor substrate 510.

The deep trenches 719 may define areas for subpixels 800. Each subpixel800 may be located within a respective subpixel region, which is locatedwithin a region of a pixel, i.e., within a pixel region. For example, anarea of a pixel may include an area of a first subpixel 801, an area ofa second subpixel 802, and an area of a third subpixel 803. In anillustrative example, the first subpixel 801 may be formed in a regionthat includes a photosensor configured to detect green light, the secondsubpixel 802 may be formed in a region that includes a photosensorconfigured to detect red light, and the third subpixel 803 may be formedin a region that includes a photosensor configured to detect blue light.Each subpixel 800 may include a volume containing a patterned columnarportion of the semiconductor substrate 510 that may be laterallyenclosed by a contiguously connected set of deep trenches 719. A pixelregion of a pixel includes all subpixel regions for the set of subpixels800 contained within the pixel.

Referring to FIG. 6 , the hard mask layer 712 may be removed selectiveto the semiconductor substrate 510, the pad dielectric layer 711, andthe shallow trench isolation structures 620. In an illustrative example,if the hard mask layer 712 includes silicon nitride, a wet etch processusing hot phosphoric acid may be performed to remove the hard mask layer712. Subsequently, the pad dielectric layer 711 may be removed selectiveto the semiconductor substrate 510.

Referring to FIG. 7 , a dielectric metal oxide liner 721L may beconformally deposited over the physically exposed surfaces of thesemiconductor substrate 510. The dielectric metal oxide liner 721L maybe formed on the sidewalls of the deep trenches 719, on the backsidesurface 709 of the semiconductor substrate 510, and on surfaces of theshallow trench isolation structures 620 in case the shallow trenchisolation structures 620 are physically exposed to the deep trenches719. The dielectric metal oxide liner 721L includes a dielectric metaloxide material having a dielectric constant greater than 7.9 (i.e., a“high-k” dielectric material). Other suitable high-k dielectricmaterials may also be used. For example, high-k dielectric materials mayhave a dielectric constant in a range such as from 7.9 to 100. Exemplarydielectric metal oxide materials that may be used for the dielectricmetal oxide liner 721L include hafnium oxide, aluminum oxide, zirconiumoxide, magnesium oxide, calcium oxide, yttrium oxide, tantalum oxide,strontium oxide, titanium oxide, lanthanum oxide, barium oxide orcombinations thereof. Other suitable materials are within thecontemplated scope of disclosure. The dielectric metal oxide liner 721Lmay be deposited using a chemical vapor deposition process or an atomiclayer deposition (ALD). The thickness of the dielectric metal oxideliner 721L may be in a range from 2 nm to 6 nm, although lesser andgreater thicknesses may also be used.

The dielectric metal oxide liner 721L may be formed to provide negativecharge trapping. For example, the dielectric metal oxide liner 721L maybe deposited with a non-stoichiometric oxygen-rich composition, or maybe surface-treated, for example, with plasma, to have anon-stoichiometric oxygen-rich surface compression. In this case, thedielectric metal oxide liner 721L may include an oxygen-rich dielectricmetal oxide material with negatively charged interstitial oxygen atomsand/or dangling or broken metal oxide bonds, thereby providingaccumulation of negative charges within the dielectric metal oxide liner721L. In an illustrative example, the areal density of accumulatednegative charges within the dielectric metal oxide liner 721L may be ina range from 5.0×10⁹ electrons per cm² to 1.0×10¹⁴ electrons per cm²,such as from 1.0×10¹⁰ electrons per cm² to 2.0×10¹³ electrons per cm².The dielectric metal oxide material used in the dielectric metal oxideliner 721L may accumulate more negative charges than other dielectricmaterials such as silicon nitride or silicon oxide. The negative chargein the dielectric metal oxide liner 721L increases hole accumulationwithin interfacial portions of the first-conductivity-type wells 607 andthe substrate semiconductor layer 601 of the semiconductor substrate510. A depletion region may be formed within portions of thefirst-conductivity-type wells 607 and the substrate semiconductor layer601 of the semiconductor substrate 510 that are proximal to thedielectric metal oxide liner 721L. The depletion region reduces darkcurrent and/or white pixels for the image sensor.

A dielectric isolation layer 722L may be formed by conformallydepositing a dielectric material in remaining volumes of the deeptrenches 719. The dielectric isolation layer 722L includes a dielectricmaterial such as undoped silicate glass, a doped silicate glass (such asborosilicate glass), or a combination thereof. The combination of thedielectric metal oxide liner 721L and the dielectric isolation layer722L may fill the deep trenches 719 (with or without seams and/orencapsulated cavities).

Referring to FIG. 8 , horizontal portions of the dielectric isolationlayer 722L and the dielectric metal oxide liner 721L may be removed fromabove the backside surface 709 of the semiconductor substrate 510 by aplanarization process. A recess etch and/or a chemical mechanicalplanarization process may be used to remove the horizontal portions ofthe dielectric isolation layer 722L and the dielectric metal oxide liner721L. In one embodiment, the dielectric metal oxide liner 721 may beused as an etch stop layer during a recess etch process that removes thehorizontal portions of the dielectric isolation layer 722, or as astopping layer during a chemical mechanical planarization process thatremoves the horizontal portions of the dielectric isolation layer 722.The horizontal portions of the dielectric metal oxide liner 721 thatoverlies the backside surface 709 of the semiconductor substrate 510 maybe subsequently removed by performing an isotropic etch process (such asa wet etch process) that etches the material of the dielectric metaloxide liner 721 selective to the semiconductor material of thesemiconductor substrate 510. Remaining vertically-extending portions ofthe dielectric metal oxide liner 721 and the dielectric isolation layer722 fill the deep trenches 719, and are herein referred to as deeptrench isolation structures 720.

Referring to FIG. 9 , an optional antireflective coating (ARC) layer732, an optical refraction layer 734, a dielectric grid material layer742L, and a metallic reflective material layer 744L may be sequentiallydeposited over the backside surface 709 of the semiconductor substrate510.

The optional ARC layer 732 may include an antireflective coatingmaterial that reduces reflection between the semiconductor material ofthe semiconductor substrate 510 and the overlying material layer, i.e.,the optical refraction layer 734. The optional ARC layer 732, ifpresent, may have a refractive index that is between the refractiveindex of the semiconductor material of the semiconductor substrate 510and the refractive index of the optical refraction layer 734. Theoptional ARC layer 732 may include a single material layer or a layerstack of multiple layers having gradually changing refractive indices.The optional ARC layer 732 includes an optically transparent material,and may include a semiconductor material, an insulating material, aconductive material, and/or a polymer material. The ARC layer 732 mayhave a thickness in a range from 50 nm to 300 nm, although lesser andgreater thicknesses may also be used.

The optical refraction layer 734 may include a semiconductor material(such as silicon, germanium, a silicon-germanium alloy, or a III-Vcompound semiconductor material) or a dielectric material (such assilicon oxide, silicon oxynitride, silicon nitride, or a dielectricmetal oxide (e.g., aluminum oxide). The optical refraction layer 734 mayinclude a material that is conducive to the formation of trenches havinga high aspect ratio during a subsequent anisotropic etch process. Theoptical refraction layer 734 may be formed as an un-patterned (blanket)material layer having two horizontal planar surfaces that are parallelto the backside surface 709 of the semiconductor substrate 510. Thedistal surface of the optical refraction layer 734 may be one of the twohorizontal planar surfaces of the optical refraction layer 734 that ismore distal from the semiconductor substrate 510, i.e., a top surface ofthe optical refraction layer 734.

The dielectric grid material layer 742L may include a dielectricmaterial such as silicon oxide, a porous dielectric material, polyimide,or another dielectric material. The thickness of the dielectric gridmaterial layer may be in a range from 50 nm to 500 nm, although lesserand greater thicknesses may also be used. The metallic reflectivematerial layer 744L may include a metallic material that may providehigh reflectivity. For example, the metallic reflective material layer744L may include silver, aluminum, copper, gold, or any other highlyreflective metallic material. The thickness of the metallic reflectivematerial layer 744L may be in a range from 50 nm to 500 nm, althoughlesser and greater thicknesses may also be used.

A photoresist layer 747 may be applied over the metallic reflectivematerial layer 744L, and may be lithographically patterned to formopenings within areas of the second-conductivity-type pinned photodiodelayers 602, i.e., within the areas of the photosensors that include arespective p-n junction between the second-conductivity-type pinnedphotodiode layers 602 and the first-conductivity-type wells 607. Theareas of the transistors of the sensing circuit (such as the resettransistors 640, the source-follower transistors 650, and the selecttransistors 660) may, or may not, be covered by the photoresist layer747.

Referring to FIGS. 10A and 10B, portions of the dielectric grid materiallayer 742L and the metallic reflective material layer 744L that are notmasked by the patterned portions of the photoresist layer 747 may beetched to form openings therethrough. Remaining portions of thedielectric grid material layer 742L form a dielectric grid structure742, and remaining portions of the metallic reflective material layer744L form a metallic grid structure 744. The stack of the dielectricgrid structure 742 and the metallic grid structure 744 constitutes agrid structure 740, which is also referred to as a composite gridstructure.

The grid structure 740 may overlie peripheries of thesecond-conductivity-type pinned photodiode layers 602, and define alight collection area for each photosensor located within a respectivesubpixel 800. A pixel 900 may include a set of subpixels configured todetect light at different wavelengths. Each pixel 900 may be locatedwithin a respective pixel region, which includes a set of subpixels 800.For example, a pixel 900 may include at least one instance of a firstsubpixel 801, at least one instance of a second subpixel 802, and atleast one instance of a third subpixel 803. In the illustrated example,a pixel may include a first subpixel 801 (such as a green subpixel)located in a first subpixel region, two second subpixels 802 (such astwo red subpixels) located in two second subpixel regions, and a thirdsubpixel 803 (such as a blue subpixel) located in a third subpixelregion. Generally, a pixel 900 may include various combinations of atleast two types of subpixels 800 configured to detect light at differentwavelength ranges. Alternatively, an image sensor may be a monochromaticimage sensor including a single type of subpixels 800. In this case,each pixel 900 may include only a single subpixel 800.

Generally, a grid structure 740 comprises at least a metallic gridstructure 744 having reflective sidewalls. The grid structure 740 maycomprise a composite grid structure including a vertical stack of ametallic grid structure 744 having reflective sidewalls and a dielectricgrid structure 742. The grid structure 740 may be formed over the distalsurface of the optical refraction layer 734. The grid structure 740includes openings that overlie a respective one of the photosensors 630.The portions of the distal surface of the optical refraction layer 734that are contacted by the bottom surfaces of the grid structure areplanar distal surface portions of the distal surface of the opticalrefraction layer 734. The portions of the distal surface of the opticalrefraction layer 734 that do not contact the grid structure 740 may becollaterally vertically recessed relative to the horizontal planeincluding the bottom surface of the grid structure during theanisotropic etch process that patterns the grid structure.

The grid structure 740 may divide each subpixel 800 into a detectorregion and a sensing circuit region. For example, a first subpixel 801may include a first detector region 801D that overlies thesecond-conductivity-type pinned photodiode layer 602 of the firstsubpixel 801, and a first sensing circuit region 801S that overlies thesensing circuit (640, 650, 660) of the first subpixel 801. A secondsubpixel 802 may include a second detector region 802D that overlies thesecond-conductivity-type pinned photodiode layer 602 of the secondsubpixel 802, and a second sensing circuit region 802S that overlies thesensing circuit (640, 650, 660) of the second subpixel 802. A thirdsubpixel 803 may include a third detector region 803D that overlies thesecond-conductivity-type pinned photodiode layer 602 of the thirdsubpixel 803, and a third sensing circuit region 803S that overlies thesensing circuit (640, 650, 660) of the third subpixel 803. Generally,the set of all subpixels 800 within a pixel 900 may be arranged in anypattern that is conducive to periodic repetition of the pixels 900within an array 1000 of pixels 900.

Referring to FIGS. 11A-11C, a masking material layer 750L may bedeposited over the grid structure 740 and the optical refraction layer734. The masking material layer 750L may include a material that may beetched with local etch rate variations in a subsequent anisotropic etchprocess. For example, the masking material layer 750L may include amaterial that may be deposited with significant thickness variationsduring a deposition process, such as a physical vapor deposition processthat deposits a material with particle size variations. Additionally, oralternatively, the masking material layer 750L may include a materialthat provides an enhanced etch rate at grain boundaries. Additionally,or alternatively, the masking material may include a material thatexhibits pitting properties in the material during a subsequentanisotropic etch process. Additionally, or alternatively, the maskingmaterial may include a material that exhibits a significant etch ratedependence on local plasma conditions during a subsequent anisotropicetch process. Generally, the masking material layer 750L includes amaterial having an unstable etch rate during a subsequent etch processsuch that significant local variations in thickness develops during asubsequent anisotropic etch process.

The masking material layer 750L may include a metallic material, asemiconductor material, or a dielectric material. In case a metallicmaterial is used for the masking material layer 750L, the maskingmaterial layer 750L may include a conductive metallic liner materialsuch as TiN, TaN, or WN, or a metallic material such a Ti, Ta, or W, ora stack of at least two metallic materials. Other suitable materials arewithin the contemplated scope of disclosure. In an illustrative example,the masking material layer 750L may include a stack of a titaniumnitride layer and a titanium layer. The thickness of the horizontalportion of the masking material layer 750L may be in a range from 20 nmto 100 nm, although lesser and greater thicknesses may also be used.

A photoresist layer 757 may be applied over the masking material layer750L, and may be lithographically patterned to form openings within thedetector regions (801D, 802D, 803D) and mask areas of the sensingcircuit regions (801S, 802S, 803S). The patterned portions of thephotoresist layer 757 overlie and protect underlying portions of themasking material layer 750L during a subsequent anisotropic etchprocess. In one embodiment, edges of the patterned portions of thephotoresist layer 757 may overlie the areas of the grid structure 740.

Referring to FIGS. 12A-12C, the masking material layer 750L may beanisotropically etched by performing an anisotropic etch process, suchas a reactive ion etch process. The chemistry of the anisotropic etchprocess may be selected such that the anisotropic etch process inducesrandom variations in the etch rate across the masking material layer750L. The cause of the random variations in the etch rate may include,but are not limited to, the grain structure in the masking materiallayer 750L, local variations in the material composition, highsusceptibility of the etch rate to local plasma conditions, propensityfor pitting during the etching process, susceptibility of the plasma tolocal charge accumulation, or combinations thereof. Different regions ofthe optical refraction layer 734 may be physically exposed at differenttimes during the anisotropic etch process due to the etch ratevariations across the masking material layer 750L.

The chemistry of the anisotropic etch process can be selected such thatthe anisotropic etch process collaterally etches the material of theoptical refraction layer 734, and forms non-planar distal surfaceportions 734N including random protrusions 734R on physically exposedportions of the optical refraction layer 734. Generally, non-planardistal surface portions 734N can be formed on the optical refractionlayer 734 by anisotropically etching the masking material layer 750L andcollaterally etching top portions of the optical refraction layer 734 ata terminal step of the anisotropic etch process.

While an inset in FIG. 12A illustrates one type of verticalcross-sectional profile for random protrusions 734R in the non-planardistal surface portions 734N of the optical refraction layer 734,embodiments are expressly contemplated herein in which the randomprotrusions 734R have different vertical cross-sectional profiles.Generally, variations in the vertical cross-sectional profile in therandom protrusions 734R may be achieved by selecting the material ofwithin the masking material layer 750L and the etch chemistry used toetch the masking material layer 750L and the optical refraction layer734.

In one embodiment, trenches having different depths may be formed in theoptical refraction layer 734 by etching physically exposed portions ofthe optical refraction layer 734 at a higher etch rate than the materialof the masking material layer 750L. In other words, material portions ofthe optical refraction layer 734 that are physically exposed prior totermination of the anisotropic etch process may be etched at a higheretch rate than a material of the masking material layer 750L during theanisotropic etch process.

In one embodiment, the random protrusions 734R may have a mean lateraldimension in a range from 10 nm to 200 nm. A mean lateral dimensionrefers to a diameter of an equivalent circle having a same area as arandom protrusion 734R. In one embodiment, the random protrusions have amean vertical dimension in a range from 10 nm to 100 nm. A mean verticaldimension refers to a height of a cylindrical shape having a samehorizontal cross-sectional shape as a random protrusion 734R and havinga same volume as the random protrusion 734R.

Generally, the optical refraction layer 734 may be located on thebackside of the semiconductor substrate 510. The optical refractionlayer 734 has a first refractive index, and includes planar distalsurface portions and non-planar distal surface portions 734N includingrandom protrusions 734R. The planar distal surface portions are surfaceportions that contact the bottom surface of the grid structure 740, andmay have the same areas as the bottom surface of the grid structure 740.In one embodiment, each of the non-planar distal surface portions 734Nmay have an areal overlap with a respective one of the photosensors 630in a plan view, i.e., in a view along a direction that is perpendicularto the backside surface 709 of the semiconductor substrate 510.

Unetched portions of the masking material layer 750 l can constitute anarray of masking structures 750 after the anisotropic etch process. Inone embodiment, the array of masking structures 750 can be locatedoutside areas of the non-planar distal surface portions 734N of theoptically refraction layer 734. The array of masking structures 750 canhave sidewalls that overlie, and contact, a respective portion of a topsurface of the grid structure 740. In one embodiment, each maskingstructure 750 within the array of masking structures 750 may be locatedover, and may have an areal overlap in a plan view with, a respectiveone of the sensing circuits (640, 650, 660). In one embodiment, edges ofthe masking structures 750 may be located on the top surface of the gridstructure 740. In one embodiment, the masking structures 750 do notcontact or overlie first sidewalls of the grid structure 740 that facetoward a respective one of the openings in the grid structure 740, andcontact second sidewalls of the grid structure 740 that face away from amost proximal one of the openings in the grid structure 740. Each regionsurrounded by a contiguous set of second sidewalls of the grid structure740 may be within a respective one of the sensing circuit regions (801s, 802 s, 803 s).

FIG. 12D shows a scanning electron micrograph (SEM) of a top surface ofan optical refraction layer 734 of a test sample after formation ofrandom protrusions 734R. The random protrusions 734R in the opticalrefraction layer 734 may have random shapes and random size distributionand/or random spacing between neighboring pairs of random protrusions734R.

Referring to FIGS. 13A-13C, the exemplary structure is illustrated afterremoval of the patterned photoresist layer 757. Removal of the patternedphotoresist layer 757 may be effected, for example, by ashing.

Referring to FIG. 14 , an optically transparent layer 770 having aplanar top surface may be formed over the grid structure 740. Theoptically transparent layer 770 may be formed by depositing aself-planarizing dielectric material such as flowable oxide (FOX).Alternatively, a transparent dielectric material may be deposited andplanarized, for example, by chemical mechanical planarization to providethe optically transparent layer 770.

The optically transparent layer 770 vertically extends through theopenings in the grid structure 740, and has a second refractive indexthat is different from the first refractive index. The opticallytransparent layer 770 may be formed on the non-planar distal surfaceportions 734N of the optical refraction layer 734. Thus, the non-planarinterface between the optically transparent layer 770 and the non-planardistal surface portions 734N of the optical refraction layer 734 providerandom refraction of photons that impinge onto the underlyingphotosensor 630, and may increase the quantum efficiency of thephotosensors 630 in the image sensor.

Various color filtering materials may be applied over the opticallytransparent layer 770, and may be patterned to form various colorfilters 780. The color filters 780 may include first-type color filters781 formed within the regions of the first subpixels 801, second-typecolor filters 782 formed within the regions of the second subpixels 802,and third-type cooler filters 783 formed within the regions of the thirdsubpixels 803. The composition of each color filtering material may beselected such that light within a target wavelength ranges passesthrough the color filtering material, while light outside the targetwavelength range is absorbed by the color filtering material.

Optical lenses 790 may be formed over the color filters 780 by applyingan optically transparent material over the color filters 780 and bypatterning the optically transparent material into material portionshaving convex surfaces that are centered on a respective one of theunderlying openings within the grid structure 740.

Referring to FIGS. 15A and 15B, the carrier substrate 690 and thebonding buffer layer 689 (if present) may be detached from theinterconnect-level dielectric layers 670. The semiconductor substrate510 and the device structures thereupon may be singulated into discreteimage sensors prior to, or after, detaching the carrier substrate 690from the semiconductor substrate 510.

Generally, an array 1000 of pixels may be formed on the semiconductorsubstrate 510. Each pixel within the array 1000 of pixels comprises atleast one subpixel, and each subpixel comprise a respective photosensor630 and a respective sensing circuit (640, 650, 660) located on a frontsurface 609 of the semiconductor substrate 510. The opticallytransparent layer 770 may overlie the array of masking structures 750.

Referring to FIG. 16 , a general method of forming an image sensor isprovided according to an embodiment of the present disclosure. Referringto step 1610, photosensors 630 may be formed on a front side of asemiconductor substrate 500. Referring to step 1620, an opticalrefraction layer 734 having a first refractive index may be formed on abackside of the semiconductor substrate 510, which may be thinned afterformation of the photosensors 630 and prior to formation of the opticalrefraction layer 734. Referring to step 1630, a grid structure 740 maybe formed over a distal surface of the optical refraction layer 734,which may be a planar horizontal surface (i.e., a top surface) that isdistal from the semiconductor substrate 510. The grid structure 740includes openings that overlie a respective one of the photosensors 630.Referring to step 1640, a masking material layer 750L may be formed overthe grid structure 740 and the optical refraction layer 734. Referringto step 1650, the masking material layer 750L may be anisotropicallyetched using an anisotropic etch process that collaterally etches amaterial of the optical refraction layer 734 and forms non-planar distalsurface portions 734N including random protrusions 734R on physicallyexposed portions of the optical refraction layer 734. Referring to step1660, an optically transparent layer 770 having a second refractiveindex that is different from the first refractive index may be formed onthe non-planar distal surface portions 734N of the optical refractionlayer 734.

Referring to all drawings and according to various embodiments of thepresent disclosure, provide an optical structure and method of formingthe optical structure that manages the optical refraction layer of aimage sensor to extend the photon path of light impinging upon the imagesensor. Extension of the photon path may increase light absorption toimprove the quantum efficiency of the image sensor.

According to an embodiment of the present disclosure, an opticalstructure is provided, which comprise front side sensors 600 (maycomprise a transfer transistor 630, and may be connected to a sensecircuit including additional transistors (640, 650, 660) located on afront side of a semiconductor substrate 510; an optical refraction layer734 located on a backside of the semiconductor substrate 510, having afirst refractive index, and including planar distal surface portions andnon-planar distal surface portions 734N including random protrusions734R; a grid structure 740 located on the planar distal surface portionsand including openings that overlie the non-planar distal surfaceportions 734N; and an optically transparent layer 770 verticallyextending through the openings in the grid structure 740 and contactingthe non-planar distal surface portions 734N and having a secondrefractive index that is different from the first refractive index,thereby providing a refractive interface with the non-planar distalsurface portions 734N that refracts incident light in random directions.

According to another embodiment of the present disclosure, an imagesensor is provided, which comprises: an array 1000 of pixels 900 locatedon a semiconductor substrate 510, wherein each pixel 900 within thearray 1000 of pixels 900 comprises at least one subpixel 800, and eachsubpixel 800 comprise a respective front side sensors 600 (may comprisea transfer transistor 630, and may be connected to a sense circuitincluding additional transistors (640, 650, 660) located on a frontsurface 609 of the semiconductor substrate 510; an optical refractionlayer 734 located on a backside of the semiconductor substrate 510,having a first refractive index, and including planar distal surfaceportions and non-planar distal surface portions 734N including randomprotrusions 734R, wherein each of the non-planar distal surface portions734N has an areal overlap with a respective one of the photosensors 630in a plan view; a grid structure 740 located on the planar distalsurface portions and including openings that overlie the non-planardistal surface portions 734N; and an optically transparent layer 770vertically extending through the openings in the grid structure 740 andhaving a second refractive index that is different from the firstrefractive index.

According to another embodiment of the present disclosure, a method offorming an optical structure including the steps of forming photosensorson a front side of a semiconductor substrate 510. The method furtherincluding the step of forming an optical refraction layer 734 having afirst refractive index on a backside of the semiconductor substrate. Themethod further including the step of forming a grid structure 740including openings that overlie a respective one of the photosensorsover a distal surface of the optical refraction layer 734. The methodfurther including the step of forming a masking material layer over thegrid structure 740 and the optical refraction layer 734. The methodfurther including the step of anisotropically etching the maskingmaterial layer 750 using an anisotropic etch process that collaterallyetches a material of the optical refraction layer 734 and formsnon-planar distal surface portions including random protrusions onphysically exposed portions of the optical refraction layer 734 and thestep of forming an optically transparent layer 770 having a secondrefractive index that is different from the first refractive index onthe non-planar distal surface portions of the optical refraction layer734.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. An optical structure comprising: photosensorslocated on a front side of a semiconductor substrate; an opticalrefraction layer located on a backside of the semiconductor substrate,having a first refractive index, and including planar distal surfaceportions and non-planar distal surface portions including randomprotrusions; a grid structure located on the planar distal surfaceportions and including openings that overlie the non-planar distalsurface portions; an optically transparent layer vertically extendingthrough the openings in the grid structure and contacting the non-planardistal surface portions and having a second refractive index that isdifferent from the first refractive index, thereby providing arefractive interface with the non-planar distal surface portions thatrefracts incident light in random directions; and an array of maskingstructures located outside areas of the non-planar distal surfaceportions of the optical refraction layer, and having sidewalls thatoverlie, and contact, a respective portion of a top surface of the gridstructure, wherein the optically transparent layer overlies the array ofmasking structures.
 2. The optical structure of claim 1, wherein therandom protrusions have a mean lateral dimension in a range from 10 nmto 200 nm.
 3. The optical structure of claim 1, wherein the randomprotrusions have a mean vertical dimension in a range from 10 nm to 100nm.
 4. The optical structure of claim 1, wherein the optical refractionlayer comprises a semiconductor material or a dielectric material. 5.The optical structure of claim 1, wherein the grid structure comprisesat least a metallic grid structure having reflective sidewalls.
 6. Theoptical structure of claim 1, wherein: an entirety of the grid structurecontacts a first subset of the planar distal surface portions; and thefirst subset of the planar distal surface portions is more distal fromthe semiconductor substrate than a most distal point among thenon-planar distal surface portions is from the semiconductor substrate.7. The optical structure of claim 6, further comprising an array ofmasking structures overlying the grid structures, wherein the array ofmasking structures contacts a respective one of a second subset of theplanar distal surface portions.
 8. The optical structure of claim 7,wherein the second subset of the planar distal surface portions is moreproximal to the semiconductor substrate than the first subset of theplanar distal surface portions is to the semiconductor substrate.
 9. Theoptical structure of claim 7, wherein: the optically transparent layercontacts a first subset of sidewalls of the grid structure; and thearray of masking structures contacts a second subset of the sidewalls ofthe grid structure.
 10. An optical structure comprising: photosensorslocated on a front side of a semiconductor substrate, wherein each ofthe photosensors comprises a respective subpixel located within an arrayof pixels, and each of the photosensors is electrically connected to arespective sensing circuit located on the front side of thesemiconductor substrate; an optical refraction layer located on abackside of the semiconductor substrate, having a first refractiveindex, and including planar distal surface portions and non-planardistal surface portions including random protrusions; a grid structurelocated on the planar distal surface portions and including openingsthat overlie the non-planar distal surface portions; an opticallytransparent layer vertically extending through the openings in the gridstructure and contacting the non-planar distal surface portions andhaving a second refractive index that is different from the firstrefractive index, thereby providing a refractive interface with thenon-planar distal surface portions that refracts incident light inrandom directions; and an array of masking structures located outsideareas of the non-planar distal surface portions of the opticalrefraction layer, and having sidewalls that overlie, and contact, arespective portion of a top surface of the grid structure, wherein theoptically transparent layer overlies the array of masking structures.11. The image sensor of claim 10, wherein each masking structure withinthe array of masking structures is located over, and has an arealoverlap in a plan view with, a respective one of the sensing circuits.12. The image sensor of claim 10, wherein the masking structures do notcontact or overlie first sidewalls of the grid structure that facetoward a respective one of the openings in the grid structure, andcontact second sidewalls of the grid structure that face away from amost proximal one of the openings in the grid structure.
 13. The imagesensor of claim 10, wherein the grid structure comprises a compositegrid structure including a vertical stack of a metallic grid structurehaving reflective sidewalls and a dielectric grid structure.
 14. Theimage sensor of claim 10, further comprising: an array of color filtersoverlying the optically transparent layer; and an array of lenses thatoverlie the array of color filters.
 15. The image sensor of claim 10,further comprising a masking material layer overlying portions of thegrid structure, wherein the optically transparent layer contacts anentirety of all sidewalls of the openings in the grid structure, andedges of the masking material layer overlie a top surface of the gridstructure.
 16. The image sensor of claim 10, wherein an entirety of thegrid structure contacts a first subset of the planar distal surfaceportions, and the first subset of the planar distal surface portions ismore distal from the semiconductor substrate than a most distal pointamong the non-planar distal surface portions is from the semiconductorsubstrate.
 17. An optical structure comprising: photosensors located ona front side of a semiconductor substrate; an optical refraction layerlocated on a backside of the semiconductor substrate, having a firstrefractive index, and including planar distal surface portions andnon-planar distal surface portions including random protrusions; a gridstructure located on the planar distal surface portions and includingopenings that overlie the non-planar distal surface portions; anoptically transparent layer vertically extending through the openings inthe grid structure and contacting the non-planar distal surface portionsand having a second refractive index that is different from the firstrefractive index, thereby providing a refractive interface with thenon-planar distal surface portions that refracts incident light inrandom directions; and a masking material layer overlying portions ofthe grid structure, wherein the optically transparent layer contacts anentirety of all sidewalls of the openings in the grid structure, andedges of the masking material layer overlie a top surface of the gridstructure.
 18. The optical structure of claim 17, wherein: an entiretyof the grid structure contacts a first subset of the planar distalsurface portions; and the first subset of the planar distal surfaceportions is more distal from the semiconductor substrate than a mostdistal point among the non-planar distal surface portions is from thesemiconductor substrate.
 19. The optical structure of claim 18, furthercomprising an array of masking structures overlying the grid structures,wherein the array of masking structures contacts a respective one of asecond subset of the planar distal surface portions.
 20. The opticalstructure of claim 19, wherein the second subset of the planar distalsurface portions is more proximal to the semiconductor substrate thanthe first subset of the planar distal surface portions is to thesemiconductor substrate.